The binding of a nitrogen substituted mononitroxide congener (I) of decamethonium to membrane-bound cholinergic receptor from Torpedo californica has been monitored by means of electron spin resonance. The receptor exhibited a twofold increase in affinity for I over a period of 30 min after initial mixing. One mole of spin label I is bound per mole of cobra alpha-toxin binding site and the label is completely dissociated from the complex by the addition of excess toxin. BIBLIOGRAPHIC REFERENCES: Wee, V.T., Sinha, B.K., Taylor, P.W. and Chignell, C.F.: Interaction of spin-labeled bisquaternary ammonium ligands with acetylcholinesterase. Mol. Pharmac. 12: 667-677, 1976. Weiland, G., Georgia, B., Wee, V.T., Chignell, C.F. and Taylor, P.: Ligand interactions with cholinergic receptor enriched membranes from torpedo: Influence of agonist exposure on receptor properties. Mol. Pharmac. 12: 1091-1105, 1976.